1. Technical Field
The technical field relates to a lens barrel with which a focal length can be changed.
2. Description of the Related Art
Digital single lens cameras with which an optical image of a subject can be converted into an electrical image signal and outputted have rapidly grown in popularity in recent years. This type of digital single lens camera generally has an interchangeable lens configuration, with which lenses can be attached and removed.
A high-magnification zoom lens system is sometimes employed in an interchangeable lens unit. With a telephoto zoom lens system, because the focal length of the telephoto end is long, the overall optical length (the distance from the image plane to the apex of the lens surface nearest to the object) also tends to be longer. Accordingly, the focusing of a telephoto zoom lens system is generally done by moving a first lens group nearest to the object.
However, since the lens group nearest to the object is larger than the other lens groups, it is difficult to raise the drive speed of the lens group, which means it is harder to increase the autofocus speed.
In view of this, as disclosed in Japanese Laid-Open Patent Application 2006-10991, for example, an interchangeable lens unit has been proposed in which a linear actuator is used to drive the focus lens group disposed nearest to the image plane.
With the digital camera discussed in Patent Document 1, however, it is necessary to provide a focal plane shutter between the imaging element of the camera body and the lens group nearest to the image plane in the interchangeable lens unit, so the back focus of the optical system (the distance between the imaging sensor and the lens nearest to the image plane) has to be made longer. In this case, to accommodate a telephoto zoom lens system, the distance that the focus lens group moves in the optical axis direction must also be increased corresponding to the zoom ratio.
With a conventional method, however, since the focus lens group is movably supported by a fixed frame, to ensure enough movement of the focus lens in the optical axis direction, space for this movement has to be provided ahead of and behind the focus lens group, and this tends to make the optical system larger.
In other words, with a conventional lens barrel, it was difficult to increase the speed of autofocusing and to reduce the size at the same time.